Pavan Koli writes:

(a generally good description of an approach to the problem)

hidden from him. But if someone tries to spam the mailing list, that person can be caught by noting his anonymous id.

I'm not sure what use case you have in mind. Why would a spammer post to the anonymous list from the same address twice? If subscription (and posting) requires owner approval, such spamming is very rare anyway.

3. I didn't come across a single mailing list for whistleblowers, activists, or people trading very sensitive information.

You won't. They have alternative channels for transmitting information, just like spies employed by governments or corporations.

Mail spoofing attempts can be stopped by encrypting mails,

Encrypted lists is a different use case. You'd use digital signatures in this case.

using PGP, but there is one problem. The person encrypting the mail would have to share their public key with everyone on the mailing list, which can be a tedious task as the mailing lists keep on changing in size,

Key distribution in this case is easy. Just post it to the mailing list. :-)

and also mails can be leaked if public key falls into wrong hands.

This isn't a real use case. Think carefully about your definition of "wrong hands" in the context of "whistleblower".

I've come up with a solution for this, these mailing lists will be kept in a very different category from others. Here when ever a user will register, they'll have to also provide their public key.

This is in fact the same basic approach as a previous GSoC project which hasn't been integrated yet.

Problem- The list manager has to be authentic, using their public key list subscribers can verify their authenticity

I don't understand what you mean.

(Or I propose a public key for the list itself and then people can use it to verify lists authenticity).

I think this is the right solution anyway. One possibility would be to use DKIM signature technology (RFC 6376, I think).